The proposed research will investigate the basis of categorization by macaque monkeys. Monkeys will be given a categorization task in which they must respond discriminatively to photographs containing monkeys or humans. The basis for this differential response has been debated in the animal cognition literature. One explanation has been that the categories are predetermined by the evolutionary history of the animal, and that the basis for the response is conceptual. The opposing explanation has been that the animals performing this task respond on the basis of individual features of the photographs, classifying individual exemplars on the basis of physical similarity or stimulus generalization. Three experiments will address (1) the nature of errors during categorization to determine whether any systematic features define each category, (2) the degree to which manipulations of the stimuli will affect performance in a conceptual same-different task using category membership as the basis for the same-different judgment, and (3) whether degradation of the stimuli which eliminates information about physical identity affects the monkey's ability to make category assignments. The results of these studies have implications for nonverbal factors underlying human categorization and cognition, and will provide data that can be useful in developing a nonhuman primate model of certain human conditions such as visual agnosia.